Recently, a semiconductor laser diode is required to have high performances such as high power or long lifetime. Particularly, a multi-channel optical disc device has been developed as a large capacity and high speed information storing device of the next generation. Therefore, raising of the performance of a multi-beam semiconductor laser diode as a key device has been given attention. Under these circumstances, a laser diode with a PHS structure for improving the heat radiating characteristic and reducing mechanical and thermal stress during mounting, is proposed as means for raising the performance of the laser diode.
FIG. 17 shows a semiconductor laser diode having a PHS electrode, which is disclosed, for example, in Japanese Published Patent Application No.57-199286. A multi-layer epitaxial layer 101 which becomes an active region of the laser is disposed on an n-type GaAs substrate 100, a p-side ohmic electrode 102 is disposed on the multi-layer epitaxial layer 101 and a PHS electrode 106 is disposed on the p-side ohmic electrode 102. Further, an n-side ohmic electrode 103 is disposed on a rear surface of the substrate 100.
This laser diode is fabricated with the condition that the multi-layer epitaxial layer 101 side is adhered to a mount base in assembling the laser diode on the mount base, a so-called junction-down mounting. For this purpose, the PHS electrode 106 is formed on the multi-layer epitaxial layer 101 via the p-side ohmic electrode 102. The PHS electrode is provided so as to improve the thermal characteristic of the laser by rapidly diffusing heat which is generated in operating the laser and so as to reduce the mechanical and thermal stress in assembly. Gold plating with good heat conductivity and high durability is conventionally employed as material of the PHS electrode 106.
Brief description will be given of the reason why this laser diode has such complicated structure as shown in FIG. 17.
It has been known that optical density in a semiconductor laser diode is the highest at a light emitting facet. Therefore, the temperature rise is the largest in the vicinity of the light emitting facet and particularly, in high power output operation, the temperature rise and the increase in optical absorption occur interactively, and in the worst case, the facet is damaged. It is also reported that, when the PHS electrode is formed within the area of the multi-layer epitaxial layer, thermal and mechanical stress remains in edge parts of the PHS electrode 106, resulting in the characteristics deterioration (for example, Japanese Published Patent Application No.57-27090). Because of these two problems, the PHS electrode 106 as a radiator is desired to cover the whole surface of the multi-layer epitaxial layer 101 which becomes the active region of the laser diode. However, since gold as a material for the PHS electrode 106 is superior in its durability it is difficult to apply gold to a process in which the PHS electrode 106 is previously formed on the whole surface of a wafer and the facet is formed by cleaving as is conventional.
Therefore, this prior art device is produced in accordance with a process of FIGS. 18(a ) to 18(e). More particularly, first, as illustrated in FIG. 18(a), crystal growth is carried out to produce the multi-layer epitaxial layer 101 on the substrate 100, the n-side ohmic electrode 103 is formed on the rear surface of the substrate 100 and the p-side ohmic electrode 102 is formed on the multi-layer epitaxial layer 101. Secondly, a mask pattern is formed on the p-side ohmic electrode 102, and after the facets are formed by wet etching (FIGS. 18(b) and 18(c)), a photoresist 105 is deposited on the whole surface of the wafer. The photoresist 105 is patterned so that the whole surfaces of the respective multi-layer epitaxial layers 101 may be bottom surfaces of respective openings and the openings may be broadened in reverse trapezoidal shapes in cross-section (FIG. 18(d)), and thereafter, the PHS electrode 106 is formed so as to fill in the spaces of the trapezoidal shape with gold. Thereafter, the completed laser diodes are obtained by dividing the wafer into chips. Here, this prior art laser diode has such a complicated structure as shown in FIG. 17, because the PHS electrode 106 has an upper end that is broader not only in the resonator length direction but also in the device width direction than the lower end thereof, and the multi-layer epitaxial layer 101 is produced by etching not only resonator facet portions but also side surface portions of the devices.
A description is given of a second prior art semiconductor laser diode having a PHS electrode.
FIG. 19 is a perspective view showing a structure of the prior art semiconductor laser diode of selective PHS structure having a PHS electrode selectively buried in the rear surface side of a semiconductor substrate. In the figure, an n-type Al.sub.x Ga.sub.1-x As first cladding layer 2 is disposed on an n-type GaAs substrate 1, a p-type Al.sub.y Ga.sub.1-y As active layer 3 is disposed on the first cladding layer 2 and a p-type Al.sub.x Ga.sub.1-x As second cladding layer 4 is disposed on the active layer 3. A portion of the second cladding layer 4 is formed in a stripe shape ridge by etching. An n-type GaAs current blocking layer 5 is disposed on both sides of the striped ridge of the second cladding layer 4 to bury the stripe ridge, and a p-type GaAs contact layer 6 is disposed on the current blocking layers 5 and the stripe ridge. A concave part is produced on the rear surface of the substrate 1, and an n-side ohmic electrode 7 is provided on the internal wall of the concave part. The concave part on the rear surface of the substrate 1 is filled with a PHS electrode 8. Further, reference numeral 10 designates a thickness from the substrate facet to the side surface of the PHS electrode 8 (hereinafter referred to as facet thickness), numeral 11 designates a thickness from the active layer 4 to the side surface of the PHS electrode 8 (hereinafter referred to as remaining substrate thickness), and numeral 14 designates an emitted laser beam.
In this second prior art device, differently from the first prior art device, it is a feature that the laser diode is fabricated such that the PHS electrode 8 is selectively buried in the rear surface of the substrate 1. This second prior art laser diode is fabricated with the condition that the substrate 1 side is adhered to a mount base in assembling the laser diode on the mount base, so-called junction-up mounting. In order to improve the heat radiating characteristic in the selective PHS structure, the remaining substrate thickness 11 is desired to be as thin as possible, i.e., to be below 20 .mu.m. In addition, in order to increase the heat radiating characteristic in the vicinity of the laser emitting facet, the facet thickness 10 is desired to be as thin as possible. However, because it is required to be with least approximately 100 .mu.m so that the laser diode may be divided into chips at an improved yield, the facet thickness 10 is approximately 100 .mu.m.
In the second prior art selective PHS structure laser diode, since the laser diode is mounted in the junction-up mounting, also in fabricating an array laser having a plurality of laser diodes in an array, it is easy to form the p-side electrodes the respective laser diodes. Therefore, advantageously, it is possible to drive respective laser diodes independently.
In the prior art semiconductor laser diodes with a PHS structure fabricated as described above, there are some problems as follows. In the first prior art semiconductor laser diode of PHS structure shown in FIG. 17, after forming the facets by wet etching, it is necessary to form the PHS electrode 106 of the cross-section trapezoidal shape covering the whole surfaces of the multi-layer epitaxial layers 101. Therefore, the laser diode is not produced at an improved yield because of the complicated process, even employing a highly accurate mask alignment technique. Further, because the laser diode is mounted in junction-down mounting, in fabricating the laser array, it is difficult to form the p-side electrode for each laser diode, and it is difficult to apply the prior art device to the array laser in which each laser diode is driven independently.
On the other hand, since the second prior art semiconductor laser diode with the selective PHS structure shown in FIG. 19 is mounted in junction-up mounting, in fabricating the array laser, it is easy to form the p-side electrode for each laser diode, and the structure is suitable for the array laser in which each laser diode is driven independently. Since the PHS electrode 8 is selectively buried in the n-type GaAs substrate 1, however, it is impossible for the PHS electrode cover the entire surface of the active region, and particularly the heat radiating characteristic at the light emitting facet is not improved. In other words, if the facet thickness 10 is 10 .mu.m, the PHS electrode 8 functions to make the heat generated in the vicinity of the light emitting facet efficiently diffuse. However, it is extremely difficult to form the light emitting facet by cleavage with the remaining facet thickness of 10 .mu.m, and the facet thickness 10 is practically approximately 100 .mu.m. As a result, the heat radiating characteristic due to the PHS electrode 8 is extremely deteriorated at the light emitting facet, and it is difficult to make an laser array having a high power output and a long lifetime.